Transitional Fossils

A transitional fossil is defined as “a fossil that exhibits characteristics of both ancestry and derived forms”, in simpler terms it is a fossil which details through certain traits a point of evolution where one species is changing through adaptation. 

One such transitional fossil, and possibly the most well known is Archaeopteryx lithographica, was found in a piece of Solnhofen Limestone from Germany. Around 155 million years ago during the end of the Jurassic,  this area of Germany was a “warm and shallow sea”.  Parts of the sea were divided into lagoons which had a high salinity level creating an anoxic environment. It is due to this harsh environment that Archaeopteryx lithographica was preserved in the sediment of these lagoons due to a lack of scavengers. This has allowed for the specimen's delicate feathers to be preserved in remarkable detail. 

Many scientists consider Archaeopteryx to be the first bird, dating to as early as 150 million years ago. Its distinct wing and tail shape with consideration to its body structure have lead scientists to believe that Archaeopteryx was the first bird-like dinosaur to be “capable of ‘flight’ as we know it today”.  That is to say Archaeopteryx was capable of the sustained flight modern birds are.    

The importance of discoveries like Archaeopteryx are crucial moments, or timestamps of evolution that can help us understand how birds and dinosaurs evolved which can help us make evolutionary predictions. Additionally since evolution is heavily based on the surrounding environment, we can understand more about Archaeopteryx’s environment through observing the evolutionary traits that took place up until that time. 

The discovery of Australopithecus Afarensis, or better known as Lucy, was one of the most significant discoveries in paleoanthropology as it provided important evidence for the theory of human evolution. Lucy was found in Ethipoia in 1974 by Tom Gray and Donald Johanson which took two weeks to excavate and uncovered around 40% of the skeleton of Australopithecus Afarensis. 

Lucy comes from the Hominidae family which makes her part of the human/African ancesteral split, one key peice of evidence for this is the Lucy's bipedal locomotion, or rather walking upright insted of on all fours. Her skeleton shows that she had kneecaps which helps the knee joints to stay in place during bipedal motion. The knee joints themselves are larger, which is an evolutionary feature to cope with the added weight on the rear legs due to upright walking. There is also a noticable evolutionary trait in Lucy's spine, being slightly curved in order to help balance and accomadate the upright stance that comes with her bipedal movement. Altogether she was smaller than modern humans, weighing about 60 pounds but has very clear ancesteral similarities to us today.  

Tiktaalik was first discovered by scientists in 2004 on Ellesmere Island in the far North of the Canadian Artic. What makes Tiktaalik so important is that this transitional fossil marks a key moment in evolution where mamals would evolve from with the development of four legged animals. For humans, this is a moment where we can see a fossil of one of our oldest ancestors living around 375 million years ago.

One of the most noticable characteristics of Tiktaalik's fossil is its adaptability to aquatic and terrestrial environments. Its head was flatter with an ajustable neck and limbs and digits that bear a resemelence to early tetrapods. This fossil evidence suggests that Tiktaalik hunted for prey in shallow water and most importantly that it was able to move on land. A further trait of Tiktaalik is its gills and scales like a fish but also had eyes and nostrils on top of its head. This suggests that Tiktaalik was able to rise its head above the water similar to how a crocodile would today. This find filled a huge gap in the fossil record, and therefore gave further proof of the evolutionary theory.